The subject matter discussed in the background section should not be assumed to be prior art merely as a result of its mention in the background section. Similarly, a problem mentioned in the background section or associated with the subject matter of the background section should not be assumed to have been previously recognized in the prior art. The subject matter in the background section merely represents different approaches, which in and of themselves may also be inventions.
Some prior art patents are U.S. Pat. No. 3,554,846 R. J. BILLETT 1971, U.S. Pat. No. 4,247,346 Kazuo Maehara, 1978, U.S. Pat. No. 4,858,815 Derek A. Roberts, 1989; EP 1824738 B1, Steve Aemisegger, 2005, U.S. Pat. No. 8,070,039 B1, Stephen A. Johnson 2010, U.S. Pat. No. 8,181,841 B2, Stephen A. Johnson 2011, U.S. Pat. No. 8,376,210 B2, Stephen A. Johnson 2012.
The welding process typically involves pressing one of the two strap portions against the other strap portion, so that the two strap portions overlap one another, with a force to create pressure holding the two strap portions together. One of the two straps is rapidly moved relative to the other strap to generate friction at the area of interface between the two straps. The pressure and movement generate sufficient heat to cause the components to begin to melt. Once the two straps are melted at the point of contact, the movement of the two straps is terminated, and the two straps are allowed to cool down while under a pressure pushing the two strap portions together. As the straps cool down in this static condition, a welded joint is formed at the interface where the two strap portions contact one another. The welding process may be applied to polyester strap with 16 mm width and 1 mm thickness and breaking strength of about 650 kg, for example.
Conventionally produced welded joints in thermoplastic straps have found wide commercial acceptance in many applications. However, the welding process of creating such joints has limitations. Referring to FIG. 1, one of two straps typically, lower strap 41, is the stationary strap, is loaded with tension force P during the welding operation. The conventional welding process requires a certain period of time to melt the material in the contact area. In that period of time, portions deep within the lower strap warm up, which reduces the cool cross section and therefore dramatically lowers breaking strength of the lower portion of the strap. Consequently, most plastic strapping apparatuses do not allow the strap to be tensioned more than 35% of the breaking strength of the strap and therefore do not utilize all of the capability of this expensive strap.
There are known methods and apparatuses that attempt to locate the welding surfaces in a predetermined position, by for example, using a combination of (1) forces of inertia to increase stroke from zero to a maximum and (2) a spring return mechanism to return to the initial position. However, this method is not entirely satisfactory since spring mechanisms are not able to consistently and accurately provide the alignment required. Also, there are methods of using forces of inertia without a spring return mechanism, but the welding mechanism is far less reliable and stable without the spring return mechanism.
Also there are some devices that have a very reliable stroke adjusting mechanism, but this kind of mechanism is too heavy in weight and is expensive to produce. As a result of the weight of the very reliable stroke adjusting mechanisms, the very reliable stroke adjusting mechanisms can only be implemented in stationary strapping machines and are not suitable for a mobile, handheld, or portable strapping apparatus.
All of the above-described examples utilize the same idea of adjusting the stroke during the rotation of the driveshaft, which is still rotating is the same direction.